1. Field of the Invention
The present invention relates to the control of liquid flow, temperatures, temperature differences, or respectively, liquid levels in refrigeration cycles, preferably in connection with screw-type compressors employing two position relay valves with electromagnetic actuation.
2. Brief Description of the Background of the Invention Including Prior Art
The known valves for two level action for liquid cycle systems, in particular for refrigerant cycle systems of compressor refrigeration plants or heating pumps are conceived as blocking valves. The essential property of such a valve is the secure closing with a high sealing effect. Expensive constructions are known for this purpose, which contain springs subject to substantial wear. The fatigue of the reset spring retards the closing procedure. Upon breakage of the spring the valve fails completely, so that the defective valve in general allows the liquid to flow through fully. The lifetime achieved by mature constructions is sufficient for the requirements in the case of resolving conventional tasks. However, these valves are unsuitable for use as two level action control elements with high switching frequencies, for example in the case of an overheating control with a two point controller. Another pilot valve for main valves is constructed for high switching frequencies with low wear construction elements. Instead of the usual reset spring a permanent magnet is employed. The valve is unsuitable for the present purposes since for completion a main valve would also be required and the required additional energy is not available as compressed air for example in the case of refrigeration plants or heat pumps. A suitable main valve is not known. The expansion of the plant with such a compressed air apparatus is economically not acceptable and not feasible.
Furthermore, the stream of refrigerating agent entering into the evaporator in a compression refrigeration installation with inside tube evaporation has to be adapted to the operating point in each case such that the refrigeration medium evaporates completely and leaves the evaporator while overheated.
In order to achieve this control object thermostatic expansion valves are employed almost exclusively. In general, proportional controllers without auxiliary are employed. The parameter to be controlled is the overheating of the refrigeration medium leaving the evaporator and is determined via the two measurement values evaporation pressure, measured at the evaporator input and output point, and the refrigeration medium temperature measured at the evaporator output, which temperature is determined with a thermal system by comparing the force at a membrane or at a corrugated pipe.
The set point of the overheating is adjusted via the pretensioning of an additional spring and the spring also acts on the membrane or the corrugated pipe, such that a deviation from set point is formed. Depending on the set point deviation, a final control element is actuated via a valve plunger of the final control element, for example a valve cone face. The flow of refrigeration medium mass entering into the evaporator is influenced such that the overheating of the refrigeration medium at the evaporator output corresponds to the set point except for the remaining control deviation. Continuously operating main valves are employed in installations of high capacity and the main valves are controlled by a thermostatic expansion valve operating as a pilot valve. However, each thermostatic control valve is only adapted to a special refrigeration agent and it does not offer possibilities for interaction with supervisory control systems.
In the case of overheating controls in refrigeration cycles with thermostatic expansion valves there are frequently stability problems which can be traced to two causes. First, the control variable "overheating" starts to oscillate without damping upon passing of the critical circuit amplification, which renders this kind of control useless. This effect appears in connection with refrigeration installations, in particular where optimized, low mass evaporators or, respectively, overdimensioned expansion valves are employed. On the other hand, the final point of the evaporation of the refrigerating agent varies locally in the evaporator. These variations occur in each evaporator. If these variations pass on up into the suction line, then the sensor receives an unstable signal, which results in the so-called "hunting" and the uselessness of the control system.
In many cases, where the critical circuit amplification occurred, in the past the time constants of the sensors were varied and the cycle amplification, which depends on the size of the valve, was varied, in order to achieve a stabilization.
Furthermore, a certain magnitude of overheating, a so-called minimum-stable signal, has to be set as a necessary condition for functioning. In order to keep this as low as possible measures have been proposed such as for example the introduction of gauze into the evaporator pipes, which are designed to reduce the size of variation of the end point of the evaporation.
These solutions proposed do not result in a stable overheating control in all application cases.
An injection valve with electrical sensor and the generation of the force providing a proper setting are disposed in a pressurized room, which room is equipped with a heating element, are taught in the German Patent Application Laid Open DE-OS No. 2,749,250. This construction is complicated and exhibits unfavorable dynamic properties.
A publication of the Singer Corporation presents a thermoelectric expansion valve with a heated sensor from semiconductor material, which changes its electrical resistance in case of transfer from overheated refrigeration agent to liquid refrigeration agent. This change in resistance is transformed into the change of the heating current through a bimetallic strip, which acts on the final control member.
In addition to unfavorable dynamic characteristic values, it is also disadvantageous in the construction in that the phase change at the location of the sensor is unsuitable as a measurement value because of the oil circulating in the refrigerating agent cycle.
A quasi steady overheating control is proposed In the Inventor's Certificate No. 389,366 of the Soviet Union, where in each case a resistance thermometer is disposed at the evaporator input and output and its measurement values are fed to a thermostat. The overheating of the refrigeration agent at the evaporator output and the control deviation are determined from theses values. An electrical interruptor continuously periodic opens and closes a solenoid valve. The thermostat acts on the interruptor such that in case there is a change of the ratio opening time to closed time of the solenoid valve the overheating of the refrigeration means at the evaporator exit is adjusted.
This solution is disadvantageous in that a solenoid valve is employed as a final control member. In case of extremely high switching frequencies the durability and lifetime of such valves are insufficient. The interruptor puts the technical requirements of the construction at a high value. It is not assured that in case of a defect in the control system the final control member will pass into the closed position, which is indispensable for the safety of the plant.
In the case of large capacities electronically controlled motor adjusting valves are also employed. These are controlled by the temperature difference between the evaporator input and evaporator output section. The measurement of the temperature difference is generally performed with resistance thermometers. The expense of the controllers prevent their use in smaller and medium sized units.